1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer.
2. Description of Prior Art
A magnetic recording medium is widely employed as a recording medium such as an audio tape, a video tape and a floppy disk. The magnetic recording medium essentially comprises a nonmagnetic support and a magnetic recording layer provided thereon which comprises a binder and a ferromagnetic powder dispersed therein.
The magnetic recording medium is basically required to be at a high level in various properties such as electromagnetic conversion characteristics, running endurance and running property. Particularly, a video tape is desired to have excellent electromagnetic conversion characteristics such as high video output and high reproducibility of an original image, according to recent spread of an 8 mm-type tape recorder and the like.
For enhancing the electromagnetic conversion characteristics of the magnetic recording medium, a variety of methods are known. For example, the enhancement of electromagnetic conversion characteristics can be directly and effectively accomplished by improving properties of a ferromagnetic powder. For this reason, the ferromagnetic powder recently has been more minimized so as to make a higher density recording system possible. Moreover, as a material of the ferromagnetic powder, a conventional iron oxide has been replaced with a modified iron oxide such as Co-modified iron oxide, and recently further replaced with a ferromagnetic metal such as iron, nickel and cobalt, or a ferromagnetic metal alloy containing such metal.
Using thus improved ferromagnetic powder, it should be possible to obtain a magnetic recording medium having high electromagnetic conversion characteristics. However, it is practically difficult to prepare a magnetic recording medium which is enhanced in electromagnetic conversion characteristics in proportion to the improvement of a ferromagnetic powder. The reason is considered as follows. A ferromagnetic powder tends to decrease in the dispersibility in a binder, as its particle size is made smaller, or the ferromagnetic powder has such properties that its dispersibility tends to decrease, for example, in the order of .gamma.-iron oxide, Co-containing .gamma.-iron oxide and ferromagnetic metal powder. Therefore, it is sometimes brought about by the improvement of a ferromagnetic powder that the ferromagnetic powder is more poorly dispersed in a magnetic recording layer. Accordingly, excellent properties of the ferromagnetic powder is not shown sufficiently even by using the improved ferromagnetic powder.
In order to improve such unfavorably dispersed condition of the ferromagnetic powder in the magnetic recording layer, prolongation of period for a procedure of kneading or dispersing contained in the preparation of a magnetic paint is known. However, this prolonged procedure sometimes deteriorates properties of the ferromagnetic powder because a large shearing force is applied to the ferromagnetic powder in the kneading or dispersing procedure. Further, from the viewpoint of working efficiency, it is unfavorable to take a long period of time in the preparation of a magnetic recording medium.
For coping with the above-mentioned various drawbacks, there have been studied methods of effectively dispersing a ferromagnetic powder without drastic change of the conventional process for the preparation of a magnetic recording medium. For instance, a method of using a ferromagnetic powder having been treated with a surface treating agent such as a silane coupling agent and a method of using a component for enhancing the dispersibility of a ferromagnetic powder (i.e., dispersing agent) such as a fatty acid are known.
The present inventors have found that the ferromagnetic powder is not satisfactorily dispersed in a magnetic recording layer even in the case of utilizing the above-described methods.
In more detail, since the ferromagnetic powder having been treated with a silane coupling agent is made hydrophilic on its surface owing to the attached silane coupling agent, the dispersibility of the ferromagnetic powder in the magnetic paint is generally enhanced, but on the contrary, an affinity thereof for the resin component sometimes lowers. Hence, the ferromagnetic powder is still poorly dispersed in the magnetic recording layer. Further, the silane coupling agent is very expensive, so that the silane coupling agent is generally unsuitable for treating the ferromagnetic powder of a magnetic recording medium, from the econmical viewpoint.
A fatty acid generally contained as a lubricant in a magnetic recording layer of a magnetic recording medium also has a function of dispersing a ferromagnetic powder. Accordingly, adjustment of the amount of the fatty acid makes it possible to improve the dispersibility of a ferromagnetic powder. However, in the case of using the fatty acid as a dispersing agent, the fatty acid should be generally employed in a larger amount than that used as a lubricant to obtain a satisfactory effect. It is known that the fatty acid also serves as a plasticizer for the binder when an excessive amount thereof is employed, and as a result, the binder is inevitably plasticized when a large amount of the fatty acid is used as a dispersing agent for the ferromagnetic powder.